So far the harmful compounds such as sulfur oxides and nitrogen oxides in the exhaust gases discharged from stationary sources have been processed by two steps. For instance, in the presence of solid catalysts nitrogen oxides in the exhaust gases are reacted with ammonia (which is fed separately) to convert into nitrogen. Thereafter, by means of a wet type desulfurization process, sulfur oxides are removed in the form of sulfates. This process has been technically well matured because of high desulfurization and denitration rates and because of efficient removal of particles.
However, the above-described process is carried out in two steps so that a plurality of equipments are needed. As a result, a considerably high cost and a long period are needed to construct this process. Furthermore, the wet type desulfurization step is carried out at low temperatures ranging from 55.degree. to 60.degree. C. so that the diffusion of purified gases discharged from a stack is not satisfactory. It has a further disadvantage that after the purification process, the purified gases must be heated up by means of heat-exchange or by burning fuel gas if enough diffusion of the stack gas is requested. Moreover, the process has another defect that a large amount of water must be supplied to make up the loss of water from the wet type desulfurization equipment as steam.
The inventors made extensive studies and experiments in order to develop a novel exhaust-gas purifying process which not only has the above described advantages of the prior art process but also can substantially overcome the above-described disadvantages or defects thereof. That is, the inventors made an extensive research-and-development in order to provide a process which can maintain high desulfurization and denitration rates and the capability of removing dust particles (which is one of the advantages of the wet type processes) and which can accomplish both desulfurization and denitration by one step and can process the exhaust gases at such high temperature that the purified gases have enough diffusing force. The inventors found out that molten alkali hydrogensulfate, ammonium hydrogensulfate or mixture thereof can be satisfactorily and advantageously utilized as a reaction medium for processing the exhaust gases. The present invention is therefore based upon this discovery.